The present invention relates to a corona discharge type charging unit used for an electrophotographic type image forming apparatus, and more particularly relates to a corona discharge type charging unit in which a non-contact type saw-toothed electrode is used.
Concerning a corona discharge type charging unit used for an electrophotographic type image forming apparatus; there are provided two types charging units. One is a wire discharge type such as corotron, scorotron and dicorotron, and the other is a pin discharge type such as a pin electrode type and a saw-toothed electrode type. The latter generates a very small amount of ozone during its operation. Therefore, the latter is recently used for an electrophotographic type copier or printer. Concerning the structure of the charging unit, Japanese Patent Publication Open to Public Inspection Nos. 15272/1988 and 45999/1993 disclose a charging unit having a saw-toothed electrode section composed of a plurality of saw teeth.
The above charging units have the following disadvantages. In the case of the above wire discharge type charging unit, when a bias voltage impressed upon the discharging electrode is raised to enhance the discharging ability, it is impossible to conduct charging uniformly. In the case of the above charging unit having a saw-toothed electrode section composed of a plurality of saw teeth, the discharging ability is higher than that of the wire discharging type charging unit. However, in the corona discharge of the charging unit having a saw-toothed electrode section composed of a plurality of saw teeth, discharge can not be conducted uniformly by each saw-toothed electrode. In order to make the charging condition uniform, it is necessary to raise the bias voltage so as to increase the discharging current. Accordingly, the charging voltage can not be made to be uniform and stable.
Further, in the case of the charging unit having a saw-toothed electrode section, the following disadvantages are provided. When this charging unit is used for an image forming apparatus provided with a drum-shaped image forming body (photoreceptor drum), in the case of positioning in which the electrode section is not contacted with the surface of the photoreceptor drum, a clearance between the electrode and the photoreceptor drum surface is changed due to the fluctuation of the rotational photoreceptor drum. As a result, the charging voltage is not uniform, so that it can not be stabilized. When a collision roller is used so as to stabilize the rotational fluctuation, the mechanism of the apparatus becomes complicated, and when the photoreceptor drum is rotated, the collision roller bounds, which causes various problems.
The present invention has been accomplished to solve the above problems. A first object of the present invention is to provide a charging unit, the discharging ability of which is high, so that the charging can be conducted uniformly without raising the bias voltage impressed upon the discharging electrode.
In this connection, there is a tendency to downsize an image forming apparatus, so that the structure of the image forming apparatus becomes complicated. Accordingly, there is a demand for downsizing a corona charging unit capable of conducting corona discharge stably. In general, the corona charging unit is covered with a shield member having an opening in the direction of the photoreceptor. In the case of a conventional wire discharge type corona charging unit, an opening angle is reduced when the charging unit is downsized. Accordingly, a quantity of ion to reach the photoreceptor surface is decreased, and a distance from the shield to the corona electrode is shortened, so that a quantity of ion to flow to the shield is increased, and a quantity of electric charge given to the photoreceptor by a control grid is reduced. Further, it is impossible to charge the photoreceptor at the uniform voltage. In order to improve the charging electrode, there is provided a saw-toothed electrode recently. As a result of the experiment made by the present inventors, the following was found. Discharging ability of the saw-toothed electrode was higher than that of the wire discharging system, however, uniform discharge can not be conducted by the saw-toothed electrode. FIG. 15 is a view showing an apparatus to measure uneven charging. Measurement of uneven charging is made as follows. The corona charging unit 400 is provided with a saw-toothed electrode plate 411. The corona charging unit 400 is put into a charging condition. Under the corona charging unit 400, there is provided an aluminum plate 401 on which an insulating layer 402 is formed. At art upper position on the insulating layer 402, there is provided a tungsten wire 403, the diameter of which is 50 .mu.m, which is grounded through an ampere-meter A provided in a direction perpendicular to the longitudinal direction of the corona charging unit 400. The aluminum plate 401 having the tungsten wire 403 is moved in the longitudinal direction of the corona charging unit 400 at a speed of 5 mm/sec. By a current flowing in the tungsten wire 403, the discharging condition is measured. In this way, the unevenness of charging can be measured.
That is, FIG. 14 is a schematic illustration showing a condition of discharge conducted by the corona charging unit in which the saw-toothed electrode plate is used. In FIG. 14, the saw-toothed electrode plate 411 is a discharging electrode of the corona charging unit. When a voltage is impressed upon the saw-toothed electrode plate 411 and the control grid 415, a corona discharge having an orientation is generated between the electrode and the photoreceptor drum. Therefore, almost all ions generated by the discharge are directed from the top 411b of the saw-toothed electrode 411a to the control grid 415. Accordingly, discharging can be conducted without being affected by the opening angle, so that the charging performance is not deteriorated. Although the discharging ability of corona discharge conducted by the saw-toothed electrode is higher than the discharging ability of the wire discharging system, discharging from each saw-toothed electrode is not uniform. Especially, the following problems are caused. When a clearance D.sub.p at the top 411b of the saw-toothed electrode 411a is reduced lower than a predetermined value, an adjacent saw-toothed electrode interferes with the discharge, so that the uniformity of discharge is deteriorated. When the value of D.sub.p is increased, interference of discharge can be avoided between the adjacent teeth, however, the charging intensity of a portion of the image forming body close to the saw-toothed electrode is increased, and the charging intensity of a portion of the image forming body distant from the saw-toothed electrode is not increased. Therefore, it is difficult to charge the image forming body uniformly. In the corona charging unit in which the saw-toothed electrode plate is used, the corona discharge concentrates upon the top of the saw-toothed electrode. Accordingly, charging is not conducted uniformly. Further, when the charging unit is downsized, the electric charge leaks to the shield, so that a spark of electric charge tends to occur.
The above problems are solved by the present invention. A second object of the present invention is to provide a compact corona charging unit capable of charging uniformly without generating a large quantity of ozone, and the corona charging unit is suitably adapted to a color image forming apparatus.
Concerning the color image forming system, the following color image forming apparatus (A) and (B) are well known. The color image forming apparatus (A) is referred to as a multi-rotation system in which the processes of charging, image exposure and development are conducted for each rotation of the image forming body, and toner images of different color are superimposed on the image forming body while the image forming body is rotated by a plurality of times. The color image forming apparatus (B) is referred to as a single-rotation system in which the processes of charging, image exposure and development are conducted by a plurality of times while the image forming body is rotated by one rotation, and the toner images are superimposed within the period.
In the charging unit used for each color image forming apparatus described above, in a period of time in which charging is not required, of course, corona discharging is stopped. Accordingly, when toner powder is scattered while the previously formed toner image passes or in the process of development, the scattered toner powder is deposited on the electrode or grid, so that the charging capacity is deteriorated and the durability is lowered.
In the case of the color image forming apparatus (B), a plurality of charging units successively conduct the discharging operation, so that the stop time of corona discharge is long. When a monochromatic image is formed, only a specific charging unit is operated and other charging units are stopped, so that toner powder is deposited on the stopped charging units. Therefore, the stopped charging units are affected by the deposited toner powder.
The above problems of deposition of toner powder on the corona electrode may be caused in the transfer unit, the separation unit and the pre-transfer charging unit. Therefore, the transfer efficiency of a toner image onto a transfer sheet is lowered, which causes a problem in the separation and conveyance efficiency of transfer sheets.
The above problems have been solved by the present invention. A third object of the present invention is to provide a color image forming apparatus in which deposition of toner powder on the charging and separation units can be prevented by a very simple means so that the problems caused by the deposition of toner powder can be effectively prevented.